The present invention is drawn to a hydrocracking process for upgrading unconverted residuals and, more particularly, a process for upgrading unconverted residuals by the removal of degraded unstable polynuclear hydrocarbons thereby allowing the upgraded residual to be recycled to the hydrocracker so as to increase liquid yields while at the same time decrease fouling of the hydrocracker.
Polynuclear hydrocarbons are known to be coke precursors in hydrocracking reactions. By polynuclear hydrocarbons is meant xylene soluble hydrocarbon materials which are in effect coke precursors. The precursor effect is especially critical when the bottoms of the hydrocracked product are recycled back to the hydrocracking reactor. The bottoms, which are high in polynuclear hydrocarbons, are very refractory when subjected to further cracking. Polynuclear hydrocarbons tend to build up in the reaction zone resulting in fouling problems inside the reactor as well as the preheaters. Another disadvantageous effect resulting from recycling the bottoms having high contents of polynuclear hydrocarbons is the increase in coke deposits on the catalyst used in cracking which negatively effects the activity of the catalyst.
Naturally, it is highly desirable to provide a process for upgrading unconverted hydrocracked bottom products by removing therefrom the unstable polynuclear hydrocarbons which foul the hydrocracker reactor when the bottoms are recirculated thereto. It is particularly useful for increasing liquid yields, maintaining catalyst activity and decreasing reactor fouling.
Accordingly, it is a principal object of the present invention to provide a process for upgrading unconverted residuals.
It is a particular object of the present invention to provide a process for upgrading unconverted residuals by removing therefrom unstable polynuclear hydrocarbons.
It is a further object of the present invention to provide a process wherein the upgraded unconverted residuals are readily recycled to a hydrocracking reactor so as to increase liquid yields.
It is a still further object of the present invention to provide a process for increasing liquid yields when recycling unconverted residuals without excessive fouling of the heater tubes and hydrocracking reactor.
Further objects and advantages of the present invention will appear hereinbelow.